The leakage proof closure of cylindrical cells with an aqueous alkaline electrolyte solution has long been the objective of research and development activity, as can be demonstrated by a large number of patents. The different cell closure types of commercially available cylindrical cells released by major manufacturing companies embody the most up to date technical solutions.
The available cell closure assemblies have the following main common properties:
All of them use a cap formed from a metal sheet that is the negative terminal of cells. The cap has a disc-like central portion, and a flange portion extending towards the interior of the cylindrical can; PA1 A plastic top is used for electrically insulting the cathode from the anode and for providing a sealing gasket. In most designs, a blow out vent is provided in the plastic top that ruptures if the inner gas pressure exceeds a predetermined limit; PA1 A sealant, such as asphalt, polyamide or ethyl vinyl alcohol is provided at the inner surface of a gasket zone of the can, and provides a tight sealing between the can and the plastic top if a sufficiently large pressure is maintained between the contacting surfaces to be sealed; PA1 The upper end of the can is crimped over the flange, so that a thin portion of the plastic top provides electrical insulation between the can and the flange. The crimped metal has the task of maintaining the pressure required between the can and the critical portion of the plastic top for a leakage proof sealing.
In LR6 (North American AA) type cells manufactured by Union Carbide Corporation, the flange of the negative cap has a sharp circular edge, and the crimping pressure of the can acts on the outer side of an outer cylindrical portion of the plastic top which is supported from the inside by this edge and by the edge of a washer. The crimping step should be performed carefully, since in case of higher forces the circular edge of the flange can cut through the thin plastic wall and a short circuit will be experienced.
The pressure distribution along this thin plastic wall is uneven, and the maximum pressure is limited by the load bearing ability of the plastic material. After longer use or at elevated temperatures the resiliency of thermoplastic materials decreases, whereby the quality of the sealing can worsen.
In the closure assembly of LR6 cells manufactured by DURACELL Inc., the negative cap is separated from a resilient metal disc used to support the inner wall of the plastic outer cylinder of the plastic top pressed from outside by the crimped can. An outwardly bent portion of the disc presses the wall out in a radial direction; however, an outwardly projecting edge exerts shearing load on the wall of the plastic material. In case of higher pressures this edge might cut the thin plastic wall. The design is capable of providing radial forces only.
In the closure assembly of LR6 cells manufactured by FUJITSU Inc., the negative cap also has an outwardly and upwardly bent flange portion, and the crimping of the can bends the thin plastic wall over this portion so that the upper edge of the bent flange portion is pressed against the thin plastic wall. The edge to wall attachment between a metal and a plastic material is made in such a way that at higher load the edge might tend to cut through the plastic wall, so that such loads should be avoided. This closure assembly is capable of maintaining a radial bias only.
There are a number of other closure designs which combine the aforementioned principles only, and for that reason they also have limited long term leakage proof properties.
The problem of appropriate cell closure acquires an increased significance in mercury free cell technique, since mercury was an efficient material which reduced gassing within the cells. In mercury free cells, specific measures should be taken for reducing hydrogen formation and for recombining any evolved hydrogen gas.
It is the primary object of the present invention to provide a cell closure assembly which has improved performance; i.e. which can provide a better sealing effect and in which the hazard of cell shorts are reduced.
A further object is the improvement of the accuracy and reliability of the blow out vent which should act if the inside gas pressure exceeds a predetermined limit.
According to the invention, an electrochemical cell of the cylindrical type has been provided which comprises a metal can open at one end, an anode, a separator and a cathode, arranged concentrically in one another, an aqueous alkaline electrolyte solution, and a closure assembly hermetically closing the open end of the can. The closure assembly comprises a plastic top inserted in the open end of the can, and has an outer cylindrical zone, a lower rim, a central portion defining a central bore, and an intermediate portion interconnecting the central portion with the lower rim. A current collector nail is inserted through the central bore of the central portion of the closure, and penetrates deeply into the anode. A negative cap of a metal sheet material forms the negative terminal of the cell and closes the central bore. According to the invention, the negative cap has a flange portion ending in an inwardly bent rim. The can has a bead at a selected distance from the open end, which serves as an abutment member for the assembly when inserted into the can. The plastic top defines a conforming support surface for the inwardly bent rim, substantially at the connection zone of the outer cylindrical zone and the lower rim. The can has a gasket zone above the bead, and a sealant is provided to cover the inside surface of the can at the gasket zone. The gasket zone has a crimped end portion bent over the inwardly bent rim so that the outer cylindrical zone of the plastic top is pressed and crimped therebetween, whereby the inwardly bent rim provides a spring action pressing the outer cylindrical zone in radial direction towards the gasket zone, and in axial direction towards the bead.
In a preferred embodiment, the intermediate portion of the plastic top comprises a blow out vent provided by a section with a thinner wall thickness.
To provide perfect sealing, a sealant is placed between the central bore of the plastic top and the corresponding portion of the current collector nail.
It is preferable if the lower rim is pressed against the upper end portion of the cathode.
Further, in the preferred embodiment, the lower rim defines an upwardly tapering inner surface, the separator extends over the end zone of the anode, and the upper end zone thereof is abutted to the tapering inner surface of the lower rim.
It is good practice that the upper end of the cathode is provided with a chamfer zone having a profile at least partially conforming to the profile of the lower rim.
Yet further, in the preferred embodiment, the current collector nail has a head portion abutting the upper face of the central portion of the plastic top, and the upper face of the head portion is attached to and electrically connected with the negative cap.
The design is most useful in a manganese dioxide-zinc alkaline primary or rechargeable cell; and is important when the cell is mercury free.
The cell closed by the closure assembly according to the present invention has leakage proof properties that well exceed those of commercially available types, and these improved properties do not require sophisticated design or high cost manufacture.